Light sensitive recording materials may suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. Such degradation may occur when a fraction of the imaging light which strikes the photosensitive layer is not absorbed but passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. Light thus reflected may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter in the photosensitive element may cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. It is this effect which leads to image degradation. Silver halide based photograhic materials (including photothermographic materials) are prone to this form of image degradation since the photosensitive layers contain light scattering particles. The effect of light scatter on image quality is well documented and is described, for example, in T. H. James "The Theory of the Photographic Process", 4th Edition, Chapter 20, Macmillan 1977.
It is common practice to minimise the effects of light scatter by including a light absorbing layer within the photographic element. To be effective the absorption of this layer must be at the same wavelengths as the sensitivity of the photosensitive layer. In the case of imaging materials coated on transparent base, a light absorbing layer is frequently coated on the reverse side of the base from the photosensitive layer. Such a coating, known as an "antihalation layer", effectively prevents reflection of any light which has passed through the photosensitive layer.
A similar effect may be achieved by a light absorbing layer interposed between the photosensitive layer and the base. This construction, described as an "antihalation underlayer" is applicable to photosensitive coatings on transparent or non-transparent bases. A light absorbing substance may be incorporated into the photosensitive layer itself, in order to absorb scattered light. Substances used for this purpose are known as "acutance dyes". It is also possible to improve image quality by coating a light absorbing layer above the photosensitive layer of a wet processed photographic element. Coatings of this kind, described in U.S. Pat. Specification No. 4,312,941 prevent multiple reflections of scattered light between the internal surfaces of a photographic element.
When the wavelength of sensitivity of the imaging medium is within the visible regions of the electromagnetic spectrum it is often necessary to have the antihalation dye rendered colorless prior to viewing of the final image. If the dye is not rendered colorless, the visible antihalation dye will provide a background density or stain to the final image. Antihalation dyes can be rendered colorless by way of heat bleaching, development solution bleaching, a specific bleaching solution, or removal from the medium in a dissolving bath.
When the wavelength of sensitivity of the medium, and hence the wavelength of absorption of the antihalation dye, is outside the visible region, it is not necessary to decolorize the antihalation dye since it has no color (i.e., it is not visible). Some antihalation dyes may have absorption tails that extend into the visible region and would therefore have to be decolorized (e.g., U.S. Pat. No. 4,581,325).
It is very desirable to produce silver halide photographic elements for duplicating and contacting processes which may be handled safely in bright white light. The benefits of this include ease of working and inspection of the element during exposure and processing, and generally more pleasant working conditions for the operators. Negative acting silver halide elements can generally be made resistant to fogging in room light by making use of an accentuated low intensity reciprocity failure effect.
In the use of negative acting ultraviolet radiation sensitive photographic silver halide emulsions and elements, it is desirable to have the elements room light or white light handlable. Providing these emulsions and elements with ultraviolet radiation sensitivity below 400nm can provide an element with good room light handlability. These elements, primarily useful as black and white image forming elements, can still benefit from and often need backside antihalation layers in order to provide sharp images. Using UV absorbing antihalation dyes without any significant tail in the visible region of the electromagnetic spectrum has eliminated the need for decolorizing of the antihalation dye.
It has been discovered by applicants, however, that when half-tone images produced from such UV sensitive photographic media are used in dry etch processes, there is an adverse affect on the process from the residual antihalation dye. In the dry etch process, the photosensitive medium of this invention is repeatedly exposed through the black-and-white half-tone color separation image. These multiple exposures increase the dot sizes, minimizing on-press dot gain or correcting color balance or tone. If the UV dye is not present in the antihalation layer of the unexposed UV sensitive film, what is ordinarily observed is that the higher percentage dots expand to fill in the highlights (veiling) before there is sufficient dot gain in the low percentage and intermediate dots. Conversely if the UV dye is still present in the imaged medium after processing then subsequent exposures to duplicating, proofing or plate materials necessitates higher exposure to burn through the high UV Dmin areas which causes a shift in dot size, leading to inaccurate dot reproduction.
Thus two key requirements are 1) that the UV dye be present during the image step of the unimaged material for optimal dry etching, and 2) that the UV dye be removed during the chemical process for optimal subsequent contact exposures.
U.S. Reissue Pat. No. 30,303 describes UV absorbing dyes useful as filter dyes in photographic elements. The claims are for molecules of the type ##STR1## wherein n is 1 or 2, when n is 1, R.sub.1 and R.sub.2 are independently chosen to represent hydrogen, an alkyl group of 1 to 10 carbon atoms, or a cyclic alkyl group of 5 or 6 carbon atoms provided that R.sub.1 and R.sub.2 cannot both be hydrogen, or R.sub.1 and R.sub.2 taken together represent the atoms necessary to complete a cyclic amino group and when n is 2 at least one of R.sub.1 and R.sub.2 is alkylene and G represents an electron withdrawing group. They are incorporated in a photographic element of a suport, silver halide layer(s) and in UV filter layer containing above dye. Alternatively, they can be in the film support. The examples given refer to color negative coatings, though black and white constructions are not excluded. No mention is made of water solubilization or bleachability. Their use is to protect the film of unwanted UV exposure to give a more balanced color rendition.
Similar abilities are described in U.S. Pat. Nos. 4,307,184 and 4,756,908. The first patent refers to polymeric versions of the dye class and is fairly far removed from our proposed use. The second patent refers to particular versions of Formula I (R.sub.1 -R.sup.2 -allyl for 3, and R short chain alkyl, C.sub.1 -C.sup.3 R.sub.1 =opt. subs long chain alkyl &gt;C.sub.10 ). Advantages are claimed for improved absorption profiles, easier and more consistent dispersions obtainable and show a reduced speed loss compared to the derivatives claimed in Reissue Pat. No. 30,303. Again the main application is for the UV filter dye in the color negative constructions The dyes are hydrophobic in U.S. Pat. Nos. 4,307,184 and 4,576,908 and presumably would not be bleached in processing. In fact it is usually preferred to be nonbleachable and nondiffusing to give UV protection after processing. In U.S. Pat. No. 4,307,183 the supersensitizing combination of a polymeric version of Formula I with a methine spectral sensitizer in silver halide constructions is claimed.
Of more relevance is U.S. patent application Ser. No. [F/381] where water solubilized versions of I are claimed in direct positive (black and white) silver halide constructions. A general formula of water solubilization is used which may make it novel compared to the class and uses disclosed in Reissue Patent 30,303. The dye has to be reactively associated with the silver halide emulsion and is not disclosed specifically as a backside coating. Advantages are claimed for improved white light safety with minimal residual UV stain after processing.